This application is based on and claims the benefit of European Patent Application No. 01401963.2 filed on Jul. 20, 2001 which is incorporated by reference herein.
1. Field of the Invention
This invention relates to the field of optical communication technology. More particularly the invention relates to gain equalization in optical transmission systems.
2. Description of the Prior Art
In recent years, optically amplified transmission systems have been developed, in which digital data are transmitted by performing optical amplification and optical relay or repeat using optical transmission fiber. Wavelength division multiplexing (WDM) is used for increasing the capacity of fiber optic networks. In systems using WDM transmission, plural optical signal channels are carried over a single line, thereby each channel being assigned to a particular wavelength. Thereby gain equalization of long haul optical amplified transmissions becomes more and more difficult due to the increasing number of transmitted wavelengths.
A problem of such optically amplified transmission systems is that in each section of such systems extra losses can be induced due to ageing of components. In particular ageing of transmission fiber can lead to significant line losses. Such losses can be compensated by in-line optical amplification, e.g. using erbium doped fiber amplifiers (EDFA). The optical amplifier gain is increased according to the extra losses of the section. This extra amplifier gain however induces a strong gain distortion of the EDFA over the wavelength range. This gain distortion is in fact linear in dB versus wavelength and is about 0.2 dB per amplifier over 12 nm for a 1 dB gain change.
This gain distortion induces a degradation of the transmission that can be drastic for very long haul systems, e.g. for systems comprising more than one hundred repeaters.
Another problem of state-of-the-art optical transmission systems is the fact that the loss of a fiber span is not exactly known when cable and repeaters are assembled. Generally an EDFA is designed in function of it""s nominal expected gain. This nominal gain is calculated with an assumption of the span loss of the fiber cable, which is manufactured in parallel with the EDFAs.
Due to the fiber attenuation variation and depending on certain parameters during the manufacturing process, the actual loss of manufactured cable is somewhat different from the expected loss. Therefore, the real gain of the EDFAs can be lower or higher than the expected one, thus inducing a linear tilt in dB versus wavelength. This tilt is negative or positive, depending on whether the EDFA gain is higher or lower than the gain used to design the EDFA. Therefore, the optimization of the overall link can be carried out only after the assembly of the cable and the repeaters, that is just before the laying, and thus too late to introduce adapted gain flattening filters.
In addition, in the terrestrial links, EDFAs are composed of two stages and allow a mid-stage access. This mid-stage access is often used for inserting a Dispersion Compensating Fiber (DCF) to compensate for chromatic dispersion accumulated along the previous section. These amplifiers are designed to have a flat spectral gain in dB for a certain gain and a certain mid-stage. But in the real system, those amplifiers are rarely used at their nominal functional point and the gain is never flat. A further problem is therefore to find a way to compensate for this unflatness.
To try to solve the above problems and compensate for the gain tilt, it is already known to provide pumped Raman amplification, as described by Fuiitsu in the paper xe2x80x9cActive Gain Tilt Equalization by 1.43 or 1.48 xcexcm pumped Raman amplificationxe2x80x9d at the conference xe2x80x9cTopical meeting on optical amplifierxe2x80x9d in 1999.
This solution uses the fact that the Raman gain variation versus wavelength depends on the Raman pump wavelength. While a 1480 nm pump provides a Raman gain with positive slope, with a 1430 nm pump a Raman gain with negative slope is obtained. The amount of gain tilt is then adjusted by changing the Raman pump power. The disadvantages of this solution are the following: The Raman gain is not linear in dB versus wavelength over 25 nm (1535-1560 nm) for a pumping wavelength of 1430 nm. Therefore the gain equalization cannot be carried out on a wide optical bandwidth, but is limited to only 15 nm.
Therefore in view of the known solutions, that are not quite efficient, it is the main object of the present invention to provide an improved gain equalization system and method for use in optical communication systems which solves the above problems.
According to the invention a system for gain equalization in an optical communication system is provided.
The optical communication system comprises a fiber link with a two-stage EDFA with an inter-stage access, and in the inter-stage:
a Dispersion Compensating Fiber;
a Raman pump source in the contra-propagating way;
a Variable Optical Attenuator VOA; and
a gain flattening filter.
The Raman pump is adapted to provide a first gain slope with an opposite trend with respect to the filter and VOA, and is further adapted such that the pump power can be controlled so as to modify the gain slope.
Furthermore a method for gain equalization in an optical communication system is provided. The method comprises the steps of:
providing in the interstage of a two-stage EDFA:
a Dispersion Compensating Fiber;
a first gain slope by means of a Raman pump connected to the DCF fiber;
a second gain slope by means of a Variable Optical Attenuator (VOA) and a gain flattening filter coupled to the DCF fiber, and
controlling the gain equalization by changing the pump power of the Raman pump and the attenuation of the VOA.
Thereby, with a combined use of the VOA, the pump power and a little adaptation of the EDFA gain by the flattening filter, the EDFA gain can be maintained flat for a desirable range of input power.
These and further objects are achieved by means of an improved gain equalization system and method for use in optical communication systems as described in the attached claims, which are considered an integral part of the present description.
With this solution no impact on the Noise Figure is achieved as the VOA is increased at the same time as the raman gain increases. Therefore, the EDFA stands flat with no penalty on the Noise Figure. Moreover, this new solution does not add penalties due to the non-linear effects in the DCF fiber.
Furthermore this solution has the advantage to rely on technologies particularly feasible for terrestrial transmission. Another advantage of such systems is the possibility to modify the overall gain response from a remote terminal station when the link is installed. Since the degradation of the transmission due to ageing can be drastic for very long haul systems, it is most advantageous to use the present invention in such systems.
The invention will be described in detail in the following description of preferred embodiments with reference to the following figures, given by way of non-limiting example to illustrate the principles of the invention.